New Mechanisms Governing Local Formability In 3 ^rd Generation AHSS

Automotive OEMs are compelled by increasingly stringent global emissions standards to find economic solutions for building higher efficiency vehicles without compromising safety and ride quality. This challenge requires new advanced high strength steels (AHSS) that will significantly reduce vehicle weight and improve fuel economy. In addition to providing higher strength, these automotive sheet steels must have exceptional formability to produce reduced gauge parts with increasingly complex geometries.

Formability is comprised of two components, global and local. Global formability represents the ability of a sheet material to be deformed under various stress conditions and to be formed into a part without failure. It can be estimated using forming-limit diagrams or ductility measurements from conventional uniaxial tensile tests. However, these tests cannot reliably assess the local formability at the edges or at the internal holes of the blanks during stamping. Numerous correlations have been developed in an attempt to predict local formability such as yield strength to tensile ratio, true strain at fracture, and post uniform elongation but they are often inaccurate, especially for AHSS grades. In this paper, the local formability of a new class of 3^rd Generation AHSS developed by NanoSteel will be shown utilizing hole expansion ratio (HER) testing. Analysis of the resulting HER values indicates new factors that play an important role in local formability related to the underlying deformation mechanisms and structural transformation.